1. Field of the Invention
The present invention relates to an arresting insulator. More particularly, this invention pertains to an arresting insulator which promptly grounds lightning-originated surge voltages in power transmission lines. The insulator suppresses or cuts off the follow current of the surge arrester to prevent ground faults.
2. Description of the Related Art
Conventional long-rod type arresters provided at electric power substations or the like have resistors incorporated within their insulation containers. The resistors consist essentially of zinc oxide. Such resistors have a non-linear varistor voltage-current characteristic (V-I characteristic). The voltage at which such a resistor starts the lightning surge absorbing function can be defined as a voltage that causes a current of N milliamperes or greater (N: being a value between 1 and 10) to start flowing across the resistor. This voltage is called "reference voltage V.sub.NmA " in association with the value of N.
The axial length of the resistor along the direction of the electric field is defined as H (in millimeters). Thus, conventional resistors have characteristics described by the following equation: EQU V.sub.NmA /H=200 V/mm
The total required length of the resistor is determined by the maximum AC operating voltage and the characteristic of insulation coordination to the lightning surge. Particularly, the set value of the AC operating voltage greatly influences the design length of the resistor. For instance, to give the arresting function to the porcelain shed of a suspension insulator using a resistor with a reference voltage V.sub.1mA of 200 V/mm, with N equal to 1, the length of the resistor is calculated as shown in Table 1 given below. In this case, it is assumed that the maximum AC applied voltage is a temporary overvoltage (the maximum design overvoltage) in accordance with the JEC (Standard of the Japanese Electrotechnical Committee) 217.
TABLE 1 ______________________________________ Nominal Maximum AC Applied Required Resistor Voltage U [KV] Voltage U.sub.S Length [mm] ______________________________________ 11-154 ##STR1## ##STR2## 187-275 ##STR3## ##STR4## 500 ##STR5## ##STR6## ______________________________________
In Table 1, "U.sub.m " means the maximum operating voltage in each nominal voltage, and is generally expressed in Japan by the following equations; EQU U.ltoreq.275 KV; U.sub.m =U.times.(1.2/1.1) EQU U=500 KV; U.sub.m =525 KV or 550 KV
"k" in Table 1 is a constant set so that the resistor can withstand the temporary overvoltage, and is a compensation coefficient with V.sub.NmA as a reference. The value of k varies according to the type of the resistors.
The following is an example of the computation based on Table 1. With the nominal voltage being 66 KV, the required resistor length is 688/k mm. In general, the value of k ranges from 1.02 to 1.30. The result of substituting k=1.30 in the above expression indicates that the required resistor length is at least about 530 mm.
Suppose that a resistor with a length of 530 mm or longer is provided at the head or shed of the standard suspension insulator. In this case, in accordance with the Japanese Electric Committee Technical Report, Vol. 11, No. 220, "Insulation Design of Overhead Power Transmission Line," five or more insulators should be linked and the linked length of the insulators should be 730 mm. However, the length of the insulators in a standard suspension insulator is limited by the length of the metal caps and the length of the metal pins. In practice, the desired length of the non-linear resistors exceeds the space available within conventional suspension insulator string lengths.